Marine Vertebrates
Marine vertebrates, classified (following WoRMS) under the Kingdom Animalia, Phylum Chordata and Subphylum Vertebrata, are among the most structurally complex organisms. The four main marine superclasses and classes in Vertebrata are:
- Agnatha (jawless fishes)
- Class Cephalaspidomorphi (lampreys)
- Class Myxini (hagfishes)
- Gnathostomata
Pisces (formerly Osteichthyes (now a synonym)) (bony fishes)
Class Actinopterygii (ray-finned fishes)
Class Elasmobranchii (formerly Chondrichthyes) (cartilaginous fishes: sharks, rays and skates)
Class Holocephali (cartilaginous fishes: chimaeras, ratfishes)
Class Sarcopterygii (lobe-finned fishes: lungfishes, coelacanths)
Tetrapoda
Class Aves (birds)
Class Mammalia (mammals)
Class Reptilia (reptiles)
Superclasses Agnatha and Pisces are all a form of fish. Superclass Agnatha contains the 105 species of jawless fish such as lampreys and hagfish.
Superclass Pisces contains at least 27,712 species of bony fishes including Class Actinopterygii which contains well over 20,000 species of ray-finned fishes, Class Elasmobranchii with at least 928 species of sharks, rays and skates, Class Holocephali with at least 928 species of chimaeras, and Class Sarcopterygii which contains the lobe-finned fishes such as lungfishes and coelacanths.
Under Superclass Tetrapoda, important marine classes include Class Aves (the birds) containing at least 9,842 species, Class Mammalia (all mammals) with at least 4,835 species, and Class Reptilia (the reptiles) which contains at least 3,082 species, which are all have species common in both terrestrial and marine environments.
Superclass Agnatha
This superclass includes the "primitive" jawless fish such as lampreys and hagfishes in the Classes Cephalaspidomorphi (lampreys), Myxini (hagfishes), and Pteraspidomorphi (fossil jawless vertebrates).
Class Actinopterygii
There are tens of thousands of species of bony fishes found in both marine and freshwater environments.Note that the plural form of fish (“fishes”) is used when referring to more than one species of fish. Bony fishes come in all shapes and sizes and live in all marine zones. They range in size from the bluefin tuna that measure up to 3 m to the stout infant fish, which measures about 7 mm. Most fish are slightly endothermic, meaning that they are able to regulate the temperature of their bodies, but not to the extent seen with mammals.
Endothermic bony fishes belong to the Suborder Scombroidei which includes about 122 species such as albacores, bonitos, cutlassfishes, frostfishes, hairtails, kingfishes, scabbardfishes, seerfishes, tuna, and wahoo. Endothermy uses a lot of energy but results in greater muscle control, better nerve signals, and improved digestion.
Class Elasmobranchii
This class consists of cartilaginous fish such as sharks, rays, and skates, whose skeletal structures are made up primarily of cartilage. This class contains some of the first marine species to develop paired fins. They all have 5-7 gill slits and lack swim bladders. Their large livers hold a large volume of oil that aids in buoyancy.
Because they lack a swim bladder, most sharks must swim constantly to avoid sinking. Constant swimming also aids in maintaining the flow of water and oxygen over the gills through their mouths. Their skin is covered in denticles rather than scales giving it a rough sandpaper-like feel. They reproduce through internal fertilization.
Class Holocephali
This class contains cartilaginous fishes such as chimaeras and ratfishes.
Class Sarcopterygii
This class contains the lobe-finned fishes such as the lungfishes and the recently discovered coelacanths.
Marine Birds (Class Aves)
Marine birds are characterized by a variety of adaptations to marine conditions. These adaptations include the ability of feathers to resist water, the presence of salt glands, curved bills, and webbed feet. Many marine birds are able to dive into the water to capture prey and some birds, like penguins, are able to swim into deeper water.
The preening gland releases waxes and water repellant fats that create a protective shield on the bird to prevent water from saturating the feathers. This protective shield also keeps the bird insulated and, when combined with feathers made out of keratin, the birds are essentially waterproof. Salt glands allow marine birds to drink salt water and expel the excess salt from their bodies. Salt glands work by condensing salt from the blood into the sinuses allowing the bird to sneeze out the excess. Some marine birds push out salt directly from salt glands.
Seabirds have a wide variety of eating mechanisms. Cormorants and Anhingas use their curved bills to puncture fish. Many marine birds dig into the sand for prey. Some have a distensible pouch such as those found in pelicans, frigatebirds, and cormorants. This pouch is located between the two parts of the lower mandible and is used to drain sea water from around the fish before it is eaten. Frigatebirds are known to snatch fish from other birds. Flamingos have a beak that can filter small invertebrates, algae, and other small organisms from water; their long bills and long legs enable them to stand in shallow water while they search for food.
Many birds will fly directly above the surface and look for fish swimming below. Black skimmers fly close to the surface and pluck fish near the surface of the water. Gulls and terns fly higher to find fish and plummet from the air to snatch them out of the water. Penguins are an unusual but very familiar marine bird that dives down into the ocean to look for fish.
As well as having a multitude of hunting strategies and bill adaptations, marine birds also have many different types of legs and feet. If a bird has very short legs with webbed feet, it is probably a very powerful swimmer. Webbed feet are referred to as totipalmate feet (having webbing connecting all four toes) and partially webbed feet are referred to as palmate feet. Birds that do a lot of swimming often rely upon what is called countercurrent exchange to keep the cold blood flowing up and away from their feet and from shocking their body. The arteries in these birds run very close to veins so that they are warm enough to allow the bird to swim in very cold water. Many marine birds also have very well developed eyesight and an amazing sense of smell. Birds in the Order Procellariiformes, called the "tubenoses" (albatrosses (13 species), diving-petrels (4 species), shearwaters and petrels (66 species), and storm-petrels (21 species)) can smell food up to 30 km away.
Marine birds are important to ecosystems for many reasons including their ability to move seeds throughout the environment and their predatory roles. They are also some of the most well-known and loved creatures of the sea. From the folkloric albatross to the march of the penguins, sea birds have fascinated humans for generations. However, they are threatened greatly by loss of habitat, oil spills, entanglement in fishing nets or garbage, and the disruption in migration due to global warming.
Marine Mammals (Class Mammalia)
Marine mammals are vertebrates that have hair or fur, blubber, are warm blooded, use lungs to breathe air, bear live young, and produce milk through mammary glands. Marine mammals are very similar to land mammals with the exception of a thick layer of blubber instead of thick fur for insulation. They also typically have long bodies, which allow them to move swiftly through the water. Although they breathe air for oxygen, they are able to stay underwater for long periods of time because of their ability to hold extra oxygen in their muscles and blood. Many marine mammals have an excess amount of blood and can direct it to the most important organs when necessary during deep dives. They can also slow their heartbeat for more efficient oxygen use when diving.
The four most common groups of marine mammals include:
- Pinnipeds (Family Otariidae (sea lions and fur seals), Family Phocidae (seals), Family Odobenidae (walruses);
- Carnivores (Pinnipeds and Family Ursidae (polar bears) and Family Mustelidae (sea otters));
- Cetaceans (Suborder Mysticeti (baleen whales) and Suborder Odontoceti (toothed whales such as sperm whales, orca, dolphins, and porpoises); and
- Sirenians (Family Dugongidae (2 species of dugongs) and Family Trichechidae (3 species of manatees).
Whales are divided into the baleen whales (Suborder Mysticeti) and the toothed whales (Suborder Odontoceti - sperm whales, dolphins, porpoises, beaked whales, belugas, narwhals, etc.). Both types of whales have highly developed senses, blubber to keep warm and long bodies enabling them to swim quickly. Baleen whales have baleen plates instead of teeth composed of rigid fibers that act like a filter to catch zooplankton and phytoplankton. The upper jaws of baleen whales are long and flat. Toothed whales such as sperm whales and dolphins use teeth to catch prey like fish, octopus, and squid. Dolphins have larger brains than porpoises and porpoises have more rounded rostrums, triangular-shaped dorsal fins and spade-like teeth.
There are three families of pinnipeds which include the Family Otariidae (sea lions and fur seals), Family Phocidae (true seals), and Family Odobenidae (walruses). Although pinnipeds have blubber, they are warm blooded. They can live on land or in water. Seals can live out at sea for months at a time only returning to land for the mating season and to molt. Seals are found all over the world in coastal areas. The fur seals and sea lions are part of the eared seal family Otariidae. These animals have large flippers in the front, tiny ear lobes and webbed back legs that can rotate around. For this reason, eared seals are capable of moving quite easily when in water or on land. 
With no ears, short flippers and no rotation of the back flippers, true seals of the Phocidae family are easily distinguishable from the eared seals mostly because they look very awkward when trying to move on land. Walruses have a little hair on their body, long tusks and short thick whiskers. They are also quite a bit larger than other pinnipeds.
In the Order Sirenia, there are three species of manatees and two species of dugongs. Although dugongs have many of the characteristics of other marine mammals, they have a higher blubber ratio. Dugongs and manatees are gentle animals with large front flippers. Manatees are characterized by a round tail and can be found in Southeast U.S. coastal waters and in the coastal and inland waterways of Central America and along the northern coast of 
South America. Dugongs have a dolphin-like tail and are found discontinuously in coastal waters of east Africa from the Red Sea to northernmost South Africa, northeastern India, along the Malay peninsula, around the northern coast of Australia to New Guinea and many of the island groups of the South Pacific.
Polar bears are the largest of all carnivores that live on land and are found throughout the arctic on sea ice, islands, and coastlines. These amazing predators often feed on ringed, bearded, harp, and hooded seals. The skin of polar bears is black with a layer of dense underfur and a layer of outer fur called guard hairs that are actually transparent. Polar bears are also protected from extreme temperatures with a thick (10 cm) layer of blubber.
Marine Reptiles (Class Reptilia)
Reptiles are vertebrates in the Class Reptilia, which includes four orders: Testudines (turtles, terrapins, and tortoises), Squamata (lizards, worm lizards, and snakes), Crocodilia (crocodiles, alligators, gavials, and caimans), and Rhynchocephalia (two species of lizard-like tuataras). The majority of marine reptiles are sea turtles and sea snakes, as well as the marine iguana, and the saltwater crocodile. In general a reptile is an animal that has very strong, dry skin sometimes covered with scales. Reptiles are cold-blooded or ectothermic, use lungs to breathe, and have tough skin without feathers or hair. Although reptiles are most commonly found in tropical and desert environment, they are also found in lakes, ponds, oceans, and even on top of mountains. Because they cannot regulate their internal body temperature, they are not found in extremely cold climates.
The more than 260 species of turtles and tortoises almost all have a protective shell surrounding their body. They range in size from the huge leatherback sea turtle (whose shell consists of bones beneath thick skin), which can reach up to 2.4 m in length and can weigh 907 kg to the smallest bog turtle that measures only 11.4 cm at most. Turtles that live in water have a lighter, flatter shell than the terrestrial species.
Lizards are the most common reptile with more than 2,700 species. Lizards usually have four legs with claws, however there are exceptions such as the worm lizard that usually has no appendages at all. Lizards range from a few inches to nearly 10 feet long, are typically insectivores, and inhabit trees, shrubs, or rocks. Some lizards will eat other vertebrates and others only eat plants. The Galapagos Islands are home to the marine iguana, the only iguana of the 416 known species that ventures into the ocean.
Snakes are basically reptiles with no appendages. There are about 2,000 species of snakes in the world measuring between 10 cm to 7.6 m in length. Snakes are thought to have evolved from lizards. Unlike other reptiles, snakes lack outside ears and have eyes covered with permanent transparent scales. Snakes have adapted to almost all habitats in the world and can also be found on lakeshores and even in ocean waters. Sea snakes have evolved to be extensively adapted to a fully aquatic life, except for the genus Laticauda, which retains ancestral characteristics that allows limited movement on land. Sea snakes are found in warm coastal waters from the Indian to the Pacific Oceans. All have paddle-like tails and most have laterally compressed bodies. However, unlike fish, they do not have gills and must come to the surface regularly to breathe. Nevertheless, they are among the most completely aquatic of all air-breathing vertebrates. Among this group are also species with some of the most potent venoms of all snakes. Most sea snakes will bite only when provoked, while others are much more aggressive at certain times of the year. Currently there are 62 species of sea snakes.
The Crocodilians are comprised of 23 species and include alligators, crocodiles, gavials, and caimans. Crocodilians can measure anywhere from 1.2 m to 6.2 m long in the saltwater crocodile. Crocodilians are usually found in tropical waters, although some alligators live in temperate climates especially in the US and China. Crocodilians have long flattened tails that enable them to swim efficiently through water. They breathe through nostrils located at the top of their head.
All reptiles have a sophisticated brain, central nervous system, and lungs. Some snakes only have one lung. Reptiles also have a three chambered heart with the exception of crocodiles, which have four chambers like mammals or birds. The digestive system of reptiles differs from other vertebrates in that waste, including that from the urinary system, the sexual organs, and the digestive system, empties into a holding tank called the cloaca. In the cloaca, water can be reabsorbed into the body to be used again.
The tough skin of reptiles helps protect the animal from desiccation or drying out. Many species use the tough skin as a form of protection from other animals and for protection during mating rituals. Some reptiles have the ability to change color for camouflage, communication, or sexual attraction. They reproduce through internal fertilization. Most reptiles lay eggs, although some lizards and snakes give birth to live young. Reptile eggs contain yolk and protein and are protected by a leathery or hard shell, which allows carbon dioxide and water to be exchanged while protecting the embryo from drying out or being consumed by bacteria. Sea turtles have been known to lay 150 eggs multiple times every season. Sea turtles lay their eggs in the sand and then leave them to hatch alone. The large number of eggs is necessary because many hatchlings do not survive. They are vulnerable to predation by birds, snakes, mammals, and sharks. Their nesting areas are also vulnerable to coastal development and eggs are vulnerable to human consumption. Sea turtles that do reach maturity can live up to 120 years in the wild. Alligators also have a long life span up to 70 years.
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